ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 540 (2005) 311–323
www.elsevier.com/locate/nima

GRAPES-3—A high-density air shower array for studies on
the structure in the cosmic-ray energy spectrum near the knee
S.K. Guptaa,, Y. Aikawab, N.V. Gopalakrishnana, Y. Hayashib, N. Ikedab,N. Itob, A. Jaina, A.V. Johna, S. Karthikeyana, S. Kawakamib, T. Matsuyamab,
D.K. Mohantya, P.K. Mohantya, S.D. Morrisa, T. Nonakab, A. Oshimab,
B.S. Raoa, K.C. Ravindrana, M. Sasanob, K. Sivaprasada, B.V. Sreekantana,
H. Tanakab, S.C. Tonwara, K. Viswanathana, T. Yoshikoshiba

HECR Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, Indiab
Graduate School of Science, Osaka City University, Osaka 558-8585, Japan

Received 17 August 2004; received in revised form 1 November 2004; accepted 10 November 2004
The GRAPES-3 Collaboration
Available online 22 December 2004

Abstract
The change in the spectral index from about 2:7 to 3:1 at  3  1015 eV in the all-particle energy spectrum of
primary cosmic rays is very signiﬁcant for learning about the nature of cosmic sources of ultra-high energy particles andtheir acceleration and propagation in the galactic disk. Any observation of a ﬁne structure in the spectrum would be
important for improving our understanding of these physical processes. The GRAPES-3 air shower array has been
designed to achieve higher precision in determination of various shower parameters to enable observation of any ﬁne
structure in the energy spectrum, if it exists. The details of the shower detectors, shower trigger and the data acquisition
system are presented here along with estimates of trigger efﬁciencies from Monte Carlo simulations for primary photons(g-rays) and several nuclei.
r 2004 Elsevier B.V. All rights reserved.
PACS: 98.70.Sa; 96.40.Pq; 96.40.De; 98.70.Rz; 95.55.Vj; 13.85.Tp
Keywords: Cosmic rays; Extensive air showers; Composition; Energy spectra and interactions; Gamma-ray sources; Cosmic-ray
detectors; Cosmic-ray interactions

Corresponding author. Tel.: +91 22 2280 4545; fax: +91 22 2280 4610.

E-mail address: gupta@grapes.tifr.res.in (S.K. Gupta).
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2004.11.025

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1. Introduction
A detailed understanding of the nature ofcosmic-ray sources and processes accelerating
charged particles to ultra-high energy (UHE),
EX1015 eV (PeV), still eludes us although considerable progress [1] has been made during recent
years, on observational as well as theoretical
fronts. Shock acceleration [2] through the wellknown Fermi mechanism coupled with supernovae
seem to offer a good solution at least up to PeV
energies. Basically, the same mechanism has been
extended to higher energies by invoking shocks
progressing through stellar winds and acceleration
through multiple shocks within the galactic disk.
However, the observation of a change in the powerlaw spectral index from about 2:7 to 3:1 at anenergy 3 PeV called the knee, in the all-particle
energy spectrum provides an important check on
the details of various models [3] for particle
acceleration and propagation.
For example, a change from an acceleration
mechanism invoking supernova shock into theinterstellar medium for energies below a PeV tosupernova shock into the stellar wind of theprogenitor star for higher energies has beensuggested. Such a change would be accompanied
by a change in the composition of cosmic-ray ﬂux
around the energy of the knee. A similar change
could also be expected if the knee is mainly due toleakage of particles from the galactic disk when
their Larmor radii become comparable to thethickness of the disk. The situation is considerably
different in the case of the single-source model [4]which incorporates the effects due to a possible
occurrence of a supernova in relatively recent pastin the neighborhood of the Solar system. While asignature of the physical process responsible forthe knee is being sought in the composition, it isalso necessary to study the details of the energy
spectrum around the knee to search for some ﬁne
structure [5,6] for the same purpose.
The electrons, in an air shower is the moststudied component for getting information on theshower, particularly for generating trigger, determining arrival direction and estimating the energyof the primary particle. A measurement of thelateral (radial) density distribution of particles in ashower using an array of unshielded charged
particle detectors provides a good estimate of theobserved number of particles in the shower,
Z r2
N obs
¼
2prre ðrÞ dr
(1)
e
r1

where re ðrÞ is the density of particles at a distance r
from the core of the shower. With the core of theshower incident within the shower array, r1 isgenerally taken to be 0. The distance r2 depends on
the size of the shower array but the observed
lateral distribution function is generally extrapolated to r2 ¼ 1 to obtain the total number N tot
e ofparticles in the shower. Despite considerable
ﬂuctuations in the development of individual
showers in the atmosphere and the differences indevelopment proﬁle for different primary nuclei,
the shower size (N obs
or N totee ) is known to be agood parameter for estimating primary energy ofshowers.
Practical constraints limit the number of detectors employed in a shower array, thus providing
only a sampling of the particle density re ðrÞ at alimited number of points. Even in good shower
arrays, for example, the CASA [7], the Tibet ASg[8] and the KASCADE [9] arrays, the detection
area does not exceed a few percent of the physicalarea over which the array is spread out. However,this is an important parameter which contributes
signiﬁcantly to the accuracy of measurement ofshower size (and consequently primary energy) as
well as the arrival direction, particularly for UHE
g-ray astronomy. The shower size and therefore
the optimum density of the detectors is also
dependent on the observational level. It is quite
important to have higher density of detectors toachieve larger triggering efﬁciency for lower energy
showers. The MILAGRO [10] experiment is
attempting to achieve full particle detection
sensitivity over a large area by instrumenting a
8 m deep water pond of 80  60 m2 area with twolayers of photomultiplier tubes. On the other
hand, the ARGO-YBJ collaboration [11] is installing an array of resistive plate counters fully
covering an area of 74  78 m2 surrounded by
50% coverage over an area of 100  100 m2 :With similar considerations in mind, the
GRAPES-3 shower array [12] has been designed

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to have one of the most compact conﬁgurations ofconventional type arrays with only 8 m separation
between adjacent detectors which are arranged in asymmetric hexagonal geometry. While the ﬁnal
aim is to have 15 rings with 721 detectors, coveringa distance of up to 120 m from the center of thearray, observations have started in early 2000 with
217 detectors located within the inner 8 rings. The
GRAPES-3 experiment is located at Ooty (11:4 N
latitude, 76:7 E longitude and 2200 m altitude), a
popular mountain resort town in southern India.
The design of a typical density detector of the
GRAPES-3 array along with its performance is
described in Section 2. The details of the shower
trigger system and the main features of the dataacquisition system are also presented in thissection. A few interesting observational results
are also contained in Section 2. Detailed Monte
Carlo simulations have been carried out for
estimating the trigger efﬁciency as a function ofprimary energy for various primary nuclear
groups. Salient features of these simulations alongwith some of the signiﬁcant results are discussed in
Section 3. In view of the fact that the unshieldedcharged particle detectors of the GRAPES-3 array
detect not only electrons, muons and charged
hadrons but also high energy photons (g-rays),simulations have been carried out to estimate thecontribution of g-rays to observed ‘particle’
densities as a function of the core distance. The
details of these simulations and some of the results
are presented in Section 4 followed by a briefsummary in Section 5.

2. GRAPES-3 air shower array
In Fig. 1 a schematic layout of the GRAPES-3
array is shown. A total of 721 detectors areplanned to be installed over a period of time,depending on the availability of plastic scintillators
which are being fabricated in-house. Presently 288
detectors are operational. However here, data with
only 217 detectors has been used. Fig. 1 also shows
16 squares in the north-west region. Each of thesesquares represents a 4-layer muon tracking detector with an energy threshold of 1 GeV for verticalmuons. Each layer consists of 58 proportional

313

Fig. 1. Schematic layout for the 721 detector (open circles )
GRAPES-3 shower array of which 217 detectors (ﬁlled circles )used at present are shown. Each of the 16 squares (&)
represents a 35 m2 area muon tracking detector with
E m X1 GeV:

counters, each 6 m long with 10  10 cm2 crosssectional area. This large area, muon detector has
been designed for studies [13] on the compositionof primary cosmic rays and studies on cosmic
sources of UHE g-rays. Large area muon detector
is also required for observations on muons
produced by lower energy protons [14] which areaffected by phenomena occurring on the Sun, suchas solar ﬂares and coronal mass ejections leading
to magnetic disturbances around the Earth. The
details of the muon detector and its performancein relation to these physics objectives is discussed
elsewhere [15]. The actual physical layout of theelectron detectors, the central control room andthe halls housing the muon detector may be seen in
Fig. 2 which also shows the slight slope in the
ground in the east–west direction.
2.1. Shower detector
Each shower detector consists of 4 blocks ofplastic scintillators, each 5 cm thick and 50 
50 cm2 in area, placed inside a square aluminumtank as shown in Fig. 3. A 5 cm diameter

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314

aluminum cone to protect it from rain and heating
due to direct sunlight. The detector is mounted on
a 40 cm height stand, as shown in Fig. 3, to raise it
well above ﬂowing rain water. This arrangement
allows, a small muon telescope as shown in Fig. 3,
to be placed under the detector for single particle(muon) calibration as discussed below.
2.2. Shower detector calibration
Fig. 2. A view of the GRAPES-3 array showing electron
detectors and the central control room. Four halls housing themuon detectors are seen on the left. The detectors lined up fromthe top-right to the bottom-left in the picture, are along the East
to West direction.

Pulse
10 cm
HV
PMT

Scin. Block

As shown in Fig. 3, shower detectors arecalibrated using a small muon telescope made of
two independent scintillator pedals (each 15 
15 cm2 in area and 5 cm in thickness) placed inside
an aluminum box with a vertical separation of
5 cm between them. Each being fully sensitive tothe passage of minimum-ionizing particles, a 2fold 100 ns coincidence between the two scintillators selects almost all muons with zenith angle
yt50 : Most of these muons pass through theshower detector located above the muon telescope,thus providing the distribution of integratedcharge for the passage of minimum-ionizing
particles. Typical single-particle response for four
detectors in terms of the distribution of integratedcharge (ADC value) is shown in Fig. 4. The

Scin. Block

Muon Telescope
for Calibration

Fig. 3. Schematic of a shower detector, including the muon
telescope under the detector, used for calibration.

photomultiplier tube (PMT) model ETL9807B ismounted on top of a trapezoidal-shaped aluminum cone, with its face at a height of 60 cm above
the scintillator surface. The inner surfaces of thetank and the cone have been painted with superwhite (TiO2 ) paint to increase the efﬁciency forcollection of diffuse photons at the PMT. The
whole detector assembly is covered by a large

Fig. 4. Typical distributions of the integrated-charge (ADC
value) for single particles (muons) for 4 shower detectors.

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calibration distribution for each detector is used toconvert the signal observed in a shower to‘equivalent’ number of particles. The spatial nonuniformity in the response of the detector to single
particles arises due to differences in the scintillation efﬁciency from block to block and due togeometrical effects. However, all detectors havebeen designed to have a response, which is uniform
within 10%, over its 1 m2 area.
The high voltage for each detector PMT has
been adjusted to give a most probable ADC valueof about 25 counts for a minimum-ionizing
particle. A 15-bit dynamic range, 32 channel,
charge integrating ADC (CAEN model C205N)
used in the experiment allows, measurement of a
maximum density of 1000 particles. However,
the response of the PMT output becomes nonlinear for signals \200 particles. However, thislimitation in the dynamic range of the PMTs does
not affect measurement of showers of energies
t10 PeV except for detectors located within10 m from the shower core.
The arrival time of PMT output is measured toa precision of 0.3 ns, the uncertainty in the arrival
time due to photon collection at the PMT is 1 ns.
However, the time resolution is dominated by the
spread in the arrival time of shower particles at thedetector which is 3 ns. This limitation is largely
overcome by a high density of deployment ofdetectors to achieve a good angular resolution(t1 ) for showers of energy 100 TeV ð1 TeV ¼
1012 eVÞ: The accuracy in angle determination
improves with increasing primary energy.
2.3. Shower trigger
The anode pulse from each shower detector istaken to the central control room using a 230 mlong, 50 O impedance RG-58 cable. It is thenpassively split into 2 pulses, ﬁrst pulse is attenuated down to 3% of its original amplitude andthen a.c. coupled to a channel of ADC module
through an additional cable delay (80 m, 50 O
impedance RG-174). The second pulse is ampliﬁed(16-channel, Phillips Scientiﬁc ampliﬁer, Model
776) by a factor of 10 before being fed to adiscriminator (16-channel, Phillips Scientiﬁc Model 706) set at a threshold of 30 mV: One of the315

NIM outputs of the discriminator goes to achannel of the TDC (16-channel, Hoshin Model
C021 or Phillips Scientiﬁc Model 7186) while theother output goes for shower selection as well as to
the rate monitoring system (Fig. 5).
In order to achieve a low-energy threshold forthe trigger, a simple 3-line coincidence has beenused to generate the Level-0 trigger, as shown in
Fig. 6, which acts as GATE for the ADCs and
START for the TDCs. Looking at the detectors
lined up in the north–south direction, the signals
from the discriminators for all the detectors in agiven line are OR‘ed together. Then a 100 ns wide,
3-fold coincidence is generated among the lineOR‘s for each combination of three adjacent lines.
All these 3-line coincidences, (L1.L2.L3),(L2.L3.L4), (L3.L4.L5), etc., are OR‘ed together
to generate the Level-0 trigger. Note that only the
inner 6 rings (detectors 001 to 127) of the 8 rings
have been used for generating the line OR‘s inorder to optimize the selection of showers withcores incident within the array. The observed level0 trigger rate is 52 Hz.
Despite selection of showers based on 3-line
trigger, the distribution of shower cores is expected
to be uniform because of the unrestrictive natureof this trigger as explained below. Expectedly, the3-line trigger selects a signiﬁcant number of small
but local showers and also very large showers
whose cores are far away. Therefore an additional
condition has been imposed on the shower
selection, namely a requirement that at least ‘n’
out of 127 detectors should have triggered thediscriminators within 1ms using relatively slow
EPROMS. The observed shower rate for thisLevel-1 trigger is 13 Hz for a value of n ¼ 10
used in the experiment.
To study the effect of trigger selection on the
distribution of shower cores, Monte Carlo simulations have been carried out using primary protons
in the energy range 1 TeV–3 PeV, on a power law
spectrum ð2:7Þ with zenith angle yo50 : The
simulated showers were allowed to land uniformly
over the entire area covered by the 217 detectors
which is represented by the outer hexagon in
Fig. 7. A scatter plot of the core locations of 104
showers that satisfy the requirements for shower
selection (namely Level-0 and Level-1 triggers) isARTICLE IN PRESS
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S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research A 540 (2005) 311–323
GPS

GATE
ANALOG SIGNAL

230m
80m

ADC
x217

AMP
DETECTORFAST CLEAR
DISC

START

STOP
80m

CAMAC

TDC
x217

CONTROLFAN OUTx127
LEVEL 0VETO

TRIGGERMEMORYBUFFER
GATE
GEN

VETO

x127

VETO

CLEAR
LEVEL 1
TRIGGER

TRIGGERMAIN
CONTROLx217
CALIBRATIONTRIGGERRATE

MUONMONITORDETECTORPEDESTAL
TRIGGER

DAQ
PC

Fig. 5. Schematic of the signal processing system for pulses received from the PMTs of the shower detectors to generate the signals sent
to ADCs and TDCs as well as the signals used for generating the shower trigger and rate monitoring. Basic elements of the dataacquisition system are also shown.

also shown in Fig. 7. The inner hexagon represents
the boundary of the 127 detectors used ingeneration of the trigger. Since the Level-1 trigger
does not impose any location dependent selection,
the shower cores have a uniform distribution
except for the edge effects. In the analysis of data
the drop in the selection efﬁciency towards theedge of the shower array, based on the results fromthese simulations, which are energy-dependent, aretaken into account.
2.4. Data acquisition system
The data acquisition system is a modiﬁed
CAMAC-based system using crate controllers
which are designed and fabricated in-house. A
fast trigger, within 200 ns after the arrival of ashower is required to initiate the digitization in the
ADCs and TDCs and the Level-0 trigger is used toserve this objective. In addition a large number of
local, small-size showers that get selected by theLevel-0 trigger are eliminated by the Level-1
trigger. In addition to the pulse charge recorded
for shower detectors with ADCs and the pulse
arrival time recorded with TDCs, the absolute time(local time) of the shower is also recorded to a
precision of 1 ms for each Level-1 trigger. A
10 MHz temperature-stabilized quartz crystal oscillator has been used in the real-time clock whichis corrected every second with a signal from a GPS
receiver. For each Level-1 trigger, nearly 1500
bytes of data are transferred to a 32 KB buffer of adual-memory buffer. When the ﬁrst 32 KB buffer
gets ﬁlled, the stored data is transferred into thehard disk of data acquisition PC. In the meantime
a switching signal routes the incoming data to thesecond 32 KB buffer to prevent any loss of data.
The data from the hard disk are periodically

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8th RING

6th RING

Line-OR

3-Line Coincidence
LEVEL-0 TRIGGER

Fig. 6. Schematic of the shower trigger system for generating
the Level-0 trigger using the basic 3-line coincidence fromamong the inner 127 detectors (6 rings).

317

within the next 2.5 ms if the Level-1 trigger does
not get generated.
A plot of the observed signals in the detectors
for a typical shower is shown in Fig. 8. The heightof the bar for each detector represents the numberof ‘equivalent particles’ observed in that detector.
The tallest bar nearly coincides with the locationof the shower core. The distribution of the numberof TDC channels triggered for Level-1 trigger isshown in Fig. 9. Note that the mean number of
TDC channels triggered is quite large 33,
although the number required in the selection is
only 10. However, in about 0.5% of the recorded
showers, the number of TDC channels triggered iso10. This deﬁcit is due to the fact that Level-1 is aslow trigger that requires a coincidence over abroad time interval of 1 ms. In contrast the TDCs
record the arrival time of signals over a narrower
time interval of 500 ns. Therefore, if an accidental pulse, unrelated to the shower arrives at a time,outside the TDC acceptance window but inside theLevel-1 window, it would not be recorded as avalid TDC count. The observed trigger rate
0.5% of deﬁcient showers, where the numberof TDC channels is o10, is consistent withRun No. 018543 Event No. 006157
2001-04-10 14:32:41.86946

Detected Particle Number

120
100
80
60
40

E

20

S

0

Fig. 7. Core distribution of simulated proton showers(1 TeV–3 PeV, yo50 ) that trigger the system. Inner hexagon
contains 127 detectors (6 rings) used in trigger and outerhexagon contains all 217 detectors (8 rings).

25
22.5
20
17.5

N

15
12.5
10
7.5
5
2.5

transferred to CD/DVD for permanent storage.
The dead-time of the data acquisition system isonly 3.1 ms. Note that the ADCs and TDCs are
triggered by the Level-0 trigger but are fast-cleared

5

10

15

20

25

30

35

40

45

50

W

Fig. 8. A plot of the observed signals from the detectors for atypical shower. The length of the bar for each detector
represents the number of ‘equivalent’ particles observed in thedetector. The tallest bar nearly coincides with the location of
the shower core.

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D-06 (Hz)

325
323
321
319

(a)

D-16 (Hz)S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research A 540 (2005) 311–323

328
326
324

(b)

332

(c)

Temp. (C) Pres. (hPa) Trigger (Hz) D-30 (Hz)

318

328
324
13.6
13.2
788
786
784

observed individual detector counting rates of
300–2000 Hz. In addition, relevant atmosphericparameters, such as barometric pressure, temperature, humidity, rain, wind speed and wind direction are also recorded once every minute by anindependent weather station.
2.5. Rate monitoring
The stability of the response of shower detectors
is an important parameter in ensuring a stableenergy threshold for the trigger. While single
particle calibrations as discussed above (Section2.2) are taken about once a month, the counting
rate for each detector is recorded for every 100 ms
using the output pulse from the discriminatorthrough the ‘RATE MONITOR’ system (Fig. 5).
This is an independent data recording system
which has been designed to look for short termtransients and correlation of detector rates withatmospheric parameters.
In Fig. 10 variation in the rates for three of the217 detectors namely detector numbers 06, 16 and30 over a period of seven days during 15–22
February 2001 along with the variations in theshower trigger rate (Level-1), the atmospheric

(e)

25
15

(f)

5
15

Fig. 9. The observed distribution of the number of TDC
channels triggered for the Level-1 trigger.

(d)

13.4

16

17

18

19

20

21

22

Fig. 10. Observed variation in counting rates for three shower
detectors; (a) D-06, (b) D-16 and (c) D-30, recorded using therate monitoring system. The variation in; (d) the shower triggerrate, (e) the atmospheric pressure and (f) the temperature areshown for the period of seven days during 15–22 February
2001.

pressure and the temperature are shown. As
expected the temperature shows diurnal variationin phase with solar radiation received on ground as
seen in Fig. 10f. However, the pressure data as
seen from Fig. 10e shows periodic variation of two
cycles per day.
It is to be noted that the three detector rates(Figs. 10a–c) also show periodic variations of two
cycles per day, but with a second peak ofsigniﬁcantly smaller amplitude. This observation
may be understood in terms of the thermal
properties of the scintillation detectors and their
response to varying atmospheric pressure. A major
fraction of the detector rate variation seen in Fig.
10a–c is due to a negative temperature coefﬁcientof the scintillator and the PMT assembly and asmaller fraction is due to the varying absorption of
charged particles in the atmosphere. It is interesting to note that the shower trigger rate (Fig. 10d)also shows a periodic variation of two cycles per
day but in opposite sense to the pressure change(Fig. 10e).
The pressure wave is produced by a complex
interaction of solar heating and the nonlinear

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response of the upper atmosphere to this forcing.
It is well-known that the amplitude of the pressure
wave depends on the latitude of the site and islarger at lower latitudes [16]. The low latitude
location (11:4 N) of Ooty is responsible for thesizable 12 h periodicity seen in the pressure data
(Fig. 10e).
The sensitivity of the GRAPES-3 experimentcan be gauged from the fact that a small amplitude(1 hPa) of the pressure wave (0.2%) signiﬁcantly modulates the observed rate of showers(1%). For the sake of clarity the Level-1 shower
trigger rate and the pressure data are plotted
separately in Fig. 11. A strong anti-correlation
between the two parameters is clearly visible. The
pressure data shows that the highest values arereached at about 10 h and 22 h local time when theshower trigger rates reach their minimum values.
The coefﬁcient of the anti-correlation is 80%.
This anti-correlation can be explained in terms of achange in the atmospheric absorption of cosmic
ray showers due to the pressure wave. This featureof the GRAPES-3 experiment is expected to be ofcritical importance in studying various transientphenomena including those of cosmic, solar or

Trigger Rate (Hz)

13.6

(a)

13.5
13.4
13.3
13.2
13.1

Pressure (hPa)

788
787

(b)

786
785

319

atmospheric origin. The pressure effect is also
clearly seen in the muon counting rate which will
be described elsewhere [13].

3. Shower trigger efﬁciency—Monte Carlosimulations
Since one of the main objectives of the
GRAPES-3 experiment is an accurate measurement of the primary energy spectrum around theknee region and also its extension to the lowest
possible energies in order to have some overlap
with direct measurements with satellite and balloon-borne experiments [17–19], it is important tostudy the energy threshold of shower trigger, fordifferent incident primary particles under trigger
conditions described in Section 2.3. This has been
carried out using showers simulated with the
CORSIKA [20,21] package using the QGSJetmodel [22] for hadronic interactions. The detector
response in the simulations is treated by generating
an equivalent signal in each detector based on theobserved distribution for minimum-ionizing particles as shown in Fig. 4.
In order to take into account the contribution of
g-rays to the response of the 5 cm thick scintillator
used for shower detectors, showers have been
generated using the EGS package thus recording
all the electrons and g-rays (E41 MeV) reaching theobservational level of 800 g cm2 : Showers havebeen generated over a wide range of energies,
1–300 TeV, for four different types of primaryparticles, g-rays, protons, helium and iron nuclei.
The increase in the triggering efﬁciency withincreasing primary energy is shown in Fig. 12. As
expected, the efﬁciency is highest for g-ray primaries
at a given energy, reaching almost 90% at
E g 30 TeV, followed by showers initiated by protons which reach the same efﬁciency at E p 50 TeV.

784
783
15

16

17

18

19

20

21

22

Fig. 11. (a) The observed variation in shower rate and (b) theatmospheric pressure for the period of seven days during 15–22
February 2001. A 12-h periodicity is seen in the pressure and in
the shower rate data. A high degree of anti-correlation between
pressure and shower rate is clearly visible.

4. Monte Carlo simulations—response to g-rays inshowers
As mentioned above, the use of 5 cm thickplastic scintillator for the shower detector makes it
sensitive to large ﬂux of low energy g-rays which

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320

Triggering Efficiency (%)

100
γ
p
He
Fe

80

Rain Cover
PMT

60
40

Cone &
Tank

20

Concrete
Column

0

0

0.5

1

1.5

2

Scintillator
Soil
Blocks2.5

Log10 (Energy/TeV)
Fig. 12. Variation in efﬁciency of level-1 trigger as a function ofprimary energy for showers initiated by primary g-rays,
protons, helium and iron nuclei with the zenith angle, yo25
and the core inside the 6th ring of detectors. Showers have been
simulated with the CORSIKA package using the QGSJet modelfor hadron interactions and the EGS option for electrons and grays.

Fig. 13. Virtual scintillation detector of the GRAPES-3 array
constructed for use in GEANT4 package. Outer and innertrapezoidal-shaped, rain cover and cone&tank assembly respectively, are indicated by arrows. Also seen are four
scintillator blocks each 50 cm50 cm5 cm in size and the
PMT housing as indicated by arrows.

accompany the electrons in showers. Therefore theresponse of these detectors has been studied [23]using g-rays from simulated showers and the
GEANT4 package. The virtual shower detector
used in GEANT4 is shown in Fig. 13, including a
trapezoidal-shaped, aluminum cover used to protect the detector from rain. Also shown inside therain cover is a second trapezoidal-shaped aluminum cone and tank assembly which houses thefour scintillators (each 50 cm50 cm5 cm) in alight-tight environment as described in Section 2.1.
The four scintillators are clearly visible as thinsquare blocks at the bottom of the cone and tankassembly. Below the scintillators is a mild steel
structure with four legs to position the detector
40 cm above the ground. The four larger squarelegs seen near the bottom of Fig. 13 are part of aconcrete structure that anchors the detector to theground. Finally, the four large blocks seen at thebottom of Fig. 13 represent the soil below thedetector, which is also used in GEANT4 simulation to provide a realistic reproduction of thedetector and its surrounding material. A charged
particle track and its interaction products are also
shown in Fig. 13.
A total of 5  106 g-ray, 2  106 electron and2  106 muon events have been generated forstudying the response of the shower detector.
Their energies have been distributed uniformly
between 1 MeV and 10 GeV for g-rays andelectrons and between 10 MeV and 100 GeV formuons on a logarithmic scale. The particles are
randomly projected on the scintillator from a
direction with zenith angle varying from 0 to 60
and azimuthal angle from 0 to 360 : The program
determines the energy deposited in the scintillator
for each incident particle. The mean energy
deposited as a function of the energy of theincident particle is shown in Fig. 14 separately for
g-rays (top), electrons (middle) and muons (bottom), for a range of zenith angles. Each of these
distributions has been ﬁtted by a two-dimensional
function of particle energy and sec(y) as shown in
Fig. 14. These functions are then used for
calculating the detector response to the passageof g-rays, electrons and muons in a shower
simulated using the CORSIKA package as described above in Section 3.
The average lateral distribution for g-rays,
electrons and muons in showers initiated byprotons of primary energy 300 TeV, arriving invertical direction is shown in Fig. 15. It is to be
noted that the number of g-rays of energy
410 MeV at a distance of 50 m from the core is

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321

Fig. 14. The mean energy deposited as a function of the energyof the incident particle shown separately for g-rays (top),
electrons (middle) and muons (bottom), for a range of zenith
angle, expressed as sec(y).

about 5 times higher than the number of electronsof the same energy. Fig. 16 shows the averagelateral distribution in terms of the energy deposited in the scintillator due to g-rays, electrons andmuons respectively for the same set of 300 TeV
showers. This was done by using the particle
density shown in Fig. 15, and the energy deposition functions obtained from Fig. 14 separately foreach of the three components, namely g-rays,
electrons and muons.
It is seen from Fig. 16 that the energy deposited
due to muons can be easily neglected as it isrelatively small compared to the deposition due toFig. 15. Average lateral distribution for g-rays, electrons andmuons in showers initiated by vertical protons of primaryenergy of 300 TeV.

the electrons up to distances 100 m. However,
the energy deposited due to g-rays must be
properly taken into account since it adds 30%
to the energy deposited by the electrons near thecore, and increasing slowly to 70% at 100 mfrom the core. This is equivalent to a ﬂattening of
the lateral distribution of electrons which leads toan over-estimate of the shower size by 10%. The
contribution due to g-rays also depends on thenature of the primary particle. This factor has toARTICLE IN PRESS
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S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research A 540 (2005) 311–323

A comparison of the observed distribution ofthe ‘particle-sum’ over 217 detectors with the
expected distribution for showers, including thedetector response as discussed above, but only for
protons as primary particles is shown in Fig. 17. It
is interesting to note that the simulations reproduce the observations reasonably well, provided
the contribution of the low energy g-rays to thescintillation detectors is also included in carryingout the simulations.
Fig. 16. Average lateral distribution for the energy deposited in
the scintillator due to g-rays, electrons and muons obtainedusing the numbers shown in Fig. 15 and the energy deposition
functions obtained from Fig. 14.

5. Summary
The GRAPES-3 array has become operational
in March 2000 with 217 density detectors arranged
in a compact hexagonal conﬁguration with interdetector spacing of only 8 m. The 3-line trigger rateis 52 Hz which is reduced to 13 Hz when aminimum of 10 detectors are required to be
triggered in order to be able to reconstruct thearrival direction of the shower. Several of theobserved features of showers have been reproduced well, by Monte Carlo simulations whichshow that the detection energy threshold forshowers to be 50 TeV for proton primaries at
90% triggering efﬁciency. Simulation studies havealso shown that the shower size may be determinedwith an accuracy of 10% at 104:5 particles with
the GRAPES-3 array.

Acknowledgements
Fig. 17. A comparison of the observed distribution of the‘particle-sum’, over the 217 detectors for showers collected overa period of 40 days, with the distribution expected for simulated
showers for proton primaries. Showers were generated (CORSIKA with QGSJet model) assuming a differential power-law
energy spectrum, N(E)dE  E 2:7 dE, over the 1 TeV–1 PeV
energy range. The expected distribution is normalized to thetotal number of observed showers.

be considered when comparing the shower size
estimated from a simple application of the
Nishimura–Kamata–Greisen (NKG) [24,25] relation and the size determined from a complete
CORSIKA simulation using the EGS package.

We thank D.B. Arjunan, K. Manjunath, B.
Rajesh and C. Ravindran for the testing, installation and operation of various detector andelectronic modules. The administrative services of
V. Viswanathan during all phases of the experiment are gratefully acknowledged. It is a pleasure
to thank A.A. Basha, G.P. Francis, I.M. Haroon,
V. Jeyakumar, A. Pushpanathan and K. Ramadass for their help in the fabrication, assembly andinstallation of various mechanical components anddetectors. We also thank M.A. Ali, P. Amalraj, M.
Iyer, B. Jasper, G.C. Kamat, A. Pratheep, T.T.
Sreedevi and S. Thirunavukkarasu for their help incertain phases of the experiment. The JapaneseARTICLE IN PRESS
S.K. Gupta et al. / Nuclear Instruments and Methods in Physics Research A 540 (2005) 311–323

members of the GRAPES-3 collaboration wish toacknowledge the partial ﬁnancial support from the
Ministry of Education of the Government ofJapan for the experiment.
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